Fiber amplifiers and fiber lasers with reduced out-of-band gain

ABSTRACT

A method of operating a fiber amplifier characterized by a spectral gain curve includes providing an input signal at a signal wavelength. The signal wavelength lies within an in-band portion of the spectral gain curve extending from a first in-band wavelength to a second in-band wavelength, the in-band portion being characterized by a first amplitude range. The method also includes providing pump radiation at a pump wavelength. The pump wavelength is less than the signal wavelength. The method further includes coupling the pump radiation to the fiber amplifier and amplifying the input signal to generate an output signal. All portions of the spectral gain curve at wavelengths less than the first in-band wavelength and greater than the pump wavelength are characterized by a second amplitude less than or equal to 10 dB greater than the first amplitude range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/081,051 filed 06 Apr. 2011 (now U.S. Pat. No. 8,665,515), whichapplication is a division of U.S. patent application Ser. No.11/834,472, filed Aug. 6, 2007 (now U.S. Pat. No. 7,940,453), whichclaims benefit under 35 U.S.C. §119(e) of U.S. Provisional PatentApplication No. 60/836,244, filed Aug. 7, 2006, the disclosures of whichare incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of opticalamplifiers and lasers. More particularly, the present invention relatesto methods and systems related to optically excited rare-earth dopedoptical fiber gain medium. Merely by way of example, the methods andsystems have been applied to reducing out-of-band gain and amplifiedspontaneous emission in optical fibers. But it would be recognized thatthe invention has a much broader range of applicability.

Conventional laser-based material processing has generally used highpeak power pulsed lasers, for example, Q-switched Nd:YAG lasersoperating at 1064 nm, for marking, engraving, micro-machining, andcutting applications. More recently, laser systems based on fiber gainmedia have been developed. In some of these fiber-based laser systems,fiber amplifiers are utilized.

Some optical amplifiers and lasers utilizing a fiber gain medium areoptically pumped, often by using semiconductor lasers pumps. The fibergain medium is typically made of silica glass doped with rare-earthelements. The choice of the rare-earth elements and the composition ofthe fiber gain medium depends on the particular application. One suchrare-earth element is ytterbium, which is used for optical amplifiersand lasers emitting in the 1020 nm 1100 nm range. Another rare-earthelement used in some fiber gain medium is erbium, which is used foroptical amplifiers and lasers emitting in the 1530 nm-1560 nm range.

The wavelength of the optical pump source used for ytterbium-doped fiberamplifiers and lasers is typically in the wavelength range of 910 nm to980 nm. The wavelength of the optical pump source used for erbium-dopedfiber amplifiers and lasers is typically in a wavelength range centeredat about 980 nm or about 1480 nm. When ytterbium-doped or erbium-dopedfiber amplifiers are pumped at the above mentioned wavelengths, theygenerally have significant gain and amplified spontaneous emission (ASE)outside of the wavelength range of interest, i.e., the lasing oramplification wavelength. For example, when an ytterbium-doped fibergain medium is pumped at a wavelength of about 915 nm, it generates highgain and ASE at about 976 nm; when it is pumped at a wavelength ofaround 976 nm, it generates high gain and ASE at about 1030 nm. In thecase of erbium doped fiber, pumping at wavelengths of 980 nm or 1480 nmgenerates high gain and ASE at around 1530 nm.

As a result of the out-of-band gain, i.e., the gain present outside thewavelength range of interest, it is possible for the amplifiers or thelasers to produce ASE or start lasing at these out-of-band wavelengths.Such ASE or lasing will limit the amount of gain available at thewavelength of interest. In some amplifier applications, largeout-of-band ASE will limit the available gain and the ASE power may belarger than the signal power at the wavelength of interest.

Thus, there is a need in the art for fiber-based amplifiers and laserswith reduced out-of-band ASE and gain.

SUMMARY OF THE INVENTION

According to the present invention, techniques related generally to thefield of optical amplifiers and lasers are provided. More particularly,the present invention relates to a method and apparatus for amplifyingto high power laser pulses for industrial applications such as trimming,marking, cutting, and welding. Merely by way of example, the inventionhas been applied to ytterbium-doped fiber laser amplifiers. However, thepresent invention has broader applicability and can be applied to othersources.

According to an embodiment of the present invention, a method ofoperating a fiber amplifier characterized by a spectral gain curve isprovided. The method includes providing an input signal at a signalwavelength. The signal wavelength lies within an in-band portion of thespectral gain curve extending from a first in-band wavelength to asecond in-band wavelength, the in-band portion being characterized by afirst amplitude range. The method also includes providing pump radiationat a pump wavelength. The pump wavelength is less than the signalwavelength. The method further includes coupling the pump radiation tothe fiber amplifier and amplifying the input signal to generate anoutput signal. All portions of the spectral gain curve at wavelengthsless than the first in-band wavelength and greater than the pumpwavelength are characterized by a second amplitude less than or equal to10 dB greater than the first amplitude range.

According to another embodiment of the present invention, a method ofoperating an ytterbium-doped fiber amplifier is provided. The methodincludes providing an input signal at a wavelength between 1050 nm and1090 nm, providing pump radiation at a wavelength between 1010 nm and1050 nm, and coupling the pump radiation to the ytterbium-doped fiberamplifier. The method also includes amplifying the input signal togenerate an output signal.

According to yet another embodiment of the present invention, a methodof operating an ytterbium-doped fiber amplifier is provided. The methodincludes providing an input signal at a wavelength between 1050 nm and1090 nm, providing seed radiation at a wavelength between 1010 nm and1050 nm, and coupling the seed radiation to the ytterbium doped fiberamplifier. The method also includes providing pump radiation at awavelength between 910 nm and 1050 nm, coupling the pump radiation tothe fiber amplifier, and amplifying the input signal to generate anoutput signal.

According to an alternative embodiment of the present invention, amethod of operating a fiber amplifier is provided. The method includesproviding an input signal at a signal wavelength and providing pumpradiation at a pump wavelength. The pump radiation is characterized byan input pump power. The method also includes coupling the pumpradiation to the fiber amplifier. The input pump power is high enough toresult in a substantially uniform population inversion as a function offiber length.

According to a particular embodiment of the present invention, a methodof operating a fiber amplifier is provided. The method includesproviding an input signal at a signal wavelength. An in-band portion ofa spectral gain curve characterized by a first peak amplitude isassociated with the signal wavelength. The method also includesproviding pump radiation at a pump wavelength. The pump wavelength isless than the signal wavelength. The method further includes couplingthe pump radiation to the fiber amplifier and amplifying the inputsignal to generate an output signal. An out-of-band portion of thespectral gain curve characterized by a second peak amplitude andassociated with the signal wavelength is less than the in-band portionof the spectral gain curve.

According to another particular embodiment of the present invention, amethod of operating an Ytterbium-doped fiber amplifier is provided. Themethod includes providing an input signal at a wavelength between 1050nm and 1090 nm, providing pump radiation at a wavelength between 1010 nmand 1050 nm, coupling the pump radiation to the Ytterbium-doped fiberamplifier and amplifying the input signal to generate an output signal.

According to yet another particular embodiment of the present invention,a method of operating an Ytterbium-doped fiber amplifier is provided.The method includes providing an input signal at a wavelength between1050 nm and 1090 nm, providing seed radiation at a wavelength between1010 nm and 1050 nm, coupling the seed radiation to the Ytterbium-dopedfiber amplifier, providing pump radiation at a wavelength between 910 nmand 1050 nm, coupling the pump radiation to the fiber amplifier, andamplifying the input signal to generate an output signal.

According to an additional particular embodiment of the presentinvention, a method of operating a fiber amplifier is provided. Themethod includes providing an input signal at a signal wavelength andproviding pump radiation at a pump wavelength. The pump radiation ischaracterized by an input pump power. The method also includes couplingthe pump radiation to the fiber amplifier. The input pump power is highenough to result in a substantially uniform population inversion as afunction of fiber length.

According to a specific embodiment of the present invention, a method ofoperating a fiber amplifier is provided. The method includes providingan input signal at a signal wavelength and providing pump radiation at apump wavelength. The pump radiation is characterized by an input pumppower. The method also includes coupling the pump radiation to the fiberamplifier. The input pump power is high enough to result in an amount ofthe pump radiation exiting an output end of the fiber with a value atleast greater than or equal to an amount of the pump radiation beingabsorbed in the fiber.

According to another specific embodiment of the present invention, anoptical amplifier is provided. The amplifier includes a length ofrare-earth-doped fiber to amplify optical pulse signal at a firstwavelength. The optical signal wavelength is located outside of a gainpeak of the rare-earth-doped fiber. The amplifier also includes anoptical pump light at a second wavelength, which is injected into thelength of rare-earth-doped fiber. The pump light wavelength is locatednearby of a gain peak of the rare-earth-doped fiber, such that theamplified spontaneous emission and the gain at the peak aresubstantially reduced.

According to yet another specific embodiment of the present invention,an optical amplifier is provided. The amplifier includes a length ofrare-earth-doped fiber to amplify optical pulse signal at a firstwavelength. The optical signal wavelength is located outside of a gainpeak of the rare-earth-doped fiber. The amplifier also includes anoptical seed light at a second wavelength, which is injected into thelength of rare-earth-doped fiber. The seed light wavelength is locatednearby of a gain peak of the rare-earth-doped fiber. The opticalamplifier further includes an optical pump light at a third wavelength,which is also injected into the length of rare-earth-doped fiber. Thepump light wavelength is chosen such that a substantial portion of thepump light is converted to light at the seed wavelength, and such thatthe amplified spontaneous emission and the gain at the peak aresubstantially reduced.

Numerous benefits are achieved using the present invention overconventional techniques. For example, in an embodiment according to thepresent invention utilizing seed signals, optical pulses can beamplified to high powers at wavelengths outside of the natural gainpeak, with improved stability in comparison to lasers with comparableperformance characteristics. Moreover, in embodiments of the presentinvention, short pulses are generated with a reduced ASE background.Depending upon the embodiment, one or more of these benefits may exist.These and other benefits have been described throughout the presentspecification and more particularly below. Various additional objects,features and advantages of the present invention can be more fullyappreciated with reference to the detailed description and accompanyingdrawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified illustration of spectral gain in anytterbium-doped amplifier optically pumped at 976 nm;

FIG. 1B is a simplified illustration of an output spectrum of anytterbium-doped amplifier optically pumped at 976 nm;

FIG. 2 is a simplified illustration of spectral gain at various levelsof population inversion in an ytterbium-doped fiber amplifier;

FIG. 3 is a simplified schematic illustration of an optical fiberamplifier with reduced out-of-band gain according to an embodiment ofthe present invention;

FIG. 4A is a simplified illustration of a gain spectrum for an opticalfiber amplifier according to an embodiment of the present invention;

FIG. 4B is a simplified illustration of an output spectrum for anoptical fiber amplifier according to an embodiment of the presentinvention;

FIG. 5 is a simplified illustration of a gain spectrum forytterbium-doped optical fiber amplifiers of various lengths;

FIG. 6 is a simplified schematic diagram of an optical fiber amplifierwith reduced out-of-band gain according to another embodiment of thepresent invention;

FIG. 7 is a simplified illustration of spectral gain of an optical fiberamplifier with reduced out-of-band gain according to another embodimentof the present invention;

FIGS. 8A, 8B, and 8C illustrate population inversion, ASE intensity, andresidual pump power along a fiber length for a pumping power of 20 W;

FIGS. 9A, 9B, and 9C illustrate population inversion, ASE intensity, andresidual pump power along a fiber length for a pumping power of 40 W;

FIGS.10A and 10B illustrate an example of the high energy first pulseproblem and gain recovery times for a pumping power of 20 W;

FIGS. 11A and 11B illustrate an example of the high energy first pulseproblem and gain recovery times for a pumping power of 40 W;

FIG. 12 illustrates the upper-level population, the residual pump power,and the gain profile at 1028 nm along a fiber length for a pumping powerof 20 W according to an embodiment of the present invention;

FIG. 13 illustrates the upper-level population, the residual pump power,and the gain profile at 1028 nm along a fiber length for a pumping powerof 20 W according to an embodiment of the present invention; and

FIG. 14 illustrates gain and residual pump power as a function of theinjected pump power in an amplifier provided according to an embodimentof the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1A is a simplified illustration of spectral gain in anytterbium-doped amplifier optically pumped at 976 nm. To produce thespectral gain curve 110 illustrated in FIG. 1A, an ytterbium-dopedoptical fiber was core-pumped at a wavelength of 976 nm with 500 mW ofpump power. The length of optical fiber was selected to achieve close to30 dB of gain at 1064 nm. To generate the spectral gain data presentedin FIG. 1A, the inversion of the active material was about 47%. Asignificant portion of the pump light is absorbed during pumping of theytterbium amplifier.

Referring to FIG. 1A, a substantial gain peak 112 is present in thespectral gain curve at a wavelength of about 1030 nm. For applicationsin which either lasing or amplification at a wavelength of 1064 nm isdesired, the gain peak 112 at about 1030 nm represents out-of-band gain.In the example illustrated in FIG. 1A, the gain peak 112 is over 50 dB.It will be appreciated that the high value of gain peak 112 may resultin parasitic lasing or instabilities at about 1030 nm, which might limitthe gain available at 1064 nm and increase the noise at 1064 nm.Embodiments of the present invention reduce the out-of-band gain peak112, thereby providing benefits not available using conventionaltechniques.

FIG. 1B is a simplified illustration of an output spectrum 120 of anytterbium-doped amplifier optically pumped at 976 nm. Peak 122 in theoutput spectrum 120 illustrates an amplified output at the desiredwavelength of 1064 nm. However, peak 124 illustrates that substantialoutput power is present in an ASE peak 124 located at a wavelength ofabout 1030 nm. In some applications, the out-of-band power in peak 124reduces the gain at the desired wavelength of 1064 nm and results in adecrease in amplifier efficiency (e.g., output power/pump power).

FIG. 2 is a simplified illustration of spectral gain at various levelsof population inversion in an ytterbium-doped fiber amplifier. For thespectral gain curves illustrated in FIG. 2, the inversion issubstantially uniform as a function of fiber length. As a result, thegain of the amplifier is substantially uniform as a function of length.In order to generate an inversion that is substantially uniform as afunction of length, a pump source is provided that generates an amountof optical power in excess of that which can be absorbed by the activemedium in the fiber amplifier. Typically, the pump power will exceed theabsorbed power by a factor of two or more. In a specific embodiment, asdescribed below, the pump power is greater than the absorbed power by afactor of about five.

Referring to FIG. 2, curve 210 represents a population inversion of 10%,curve 212 represents a population of 20%, and curve 214 represents apopulation inversion of 30%. The remaining curves represent a populationinversion increasing by 10% per curve, with curve 222 representing apopulation inversion of 70%.

In order to generate the curves illustrated in FIG. 2, the pumpwavelength was varied from longer wavelengths, which are associated withlower inversions, to shorter wavelengths, which are associated withhigher inversions. Referring to curve 210, a pump wavelength of about1020 nm was used. The power of the pump source, the length of the fiber,the doping density, and the like were selected so that the absorption ofthe pump light was substantially uniform as a function of length,resulting in a substantially uniform inversion of about 10% along thelength of the fiber. Generally, for longer fibers, higher pump powersare provided to produce a uniform inversion.

For conditions in which the inversion is uniform along the length of thegain medium, additional increases in pump power will not result insignificant increases in absorption by the active medium or additionalinversion. Thus, the curves in FIG. 2 can be considered to as “maximuminversion” curves—the maximum inversion is reached given the wavelengthof the pump source. It will be appreciated that as the active mediumabsorbs additional pump light, the emission rate will balance theabsorption rate, resulting in a maximum absorption and a limit on thepercent of the active medium inverted.

As illustrated in FIG. 2, the choice of pump wavelength determines themaximum population inversion that can be achieved. Although thepopulation inversion may be uniform as a function of length, forexample, about 30% as illustrated by curve 214, the gain is a functionof wavelength. For pumping into a given upper-state band, this maximumpopulation inversion decreases as the pump wavelength increases. Thismaximum population inversion at a given pump wavelength dictates thevalue of the gain as a function of wavelength. When the maximumpopulation inversion is reached for any pump wavelength, the gain at thepump wavelength will be exactly 0 dB.

For example, referring to FIG. 2, at a pumping wavelength of 960 nm(curve 222), the 70% population inversion curve can be seen to giveapproximately 0 dB gain at 960 nm. Consequently, a 960 nm pumpwavelength would permit a maximum population inversion of ˜70%. Forcurve 216, with a 40% population inversion, the gain at the pumpwavelength of 985 nm is approximately 0 dB. Referring to FIG. 2, with70% inversion (curve 222), the optical gain at both 976 nm and at 1030nm is substantially larger than the optical gain at 1064 nm. As aresult, the maximum gain available to a fiber amplifier or fiber laseroperating at 1064 nm may be limited when the amplifier is pumped at 960nm.

Embodiments of the present invention provide systems that reduce theout-of-band gain with respect to the in-band gain. The particular levelsselected for the out-of-band gain and the in-band gain will depend onthe particular applications. In a particular embodiment, the out-of-bandgain is generated at a level that is negligible. In other embodiments,the out-of-band gain is merely reduced, so that it is less than or equalto 3 dB greater than the in-band gain. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives.

Referring again to FIG. 2, in the case of a ytterbium-doped opticalamplifier intended to be operated at a signal wavelength of 1064 nm(i.e., an in-band wavelength of 1064 nm), it may be advantageous tochoose a population inversion of approximately 5%-25% (generallyrepresented by curves 210-214), which reduces the out-of-band gain peaksat both 976 nm and 1030 nm that otherwise dominate at higher inversionlevels. Therefore in a specific embodiment, a pump wavelength in therange from about 1000 nm to about 1040 nm is utilized as the opticalpump in generating 1064 nm output, as determined by the 0 dB gain valuefor the 5% and 25% inversion curves.

Referring to curve 210, which has a pump wavelength of about 1020 nm,the gain at 1064 nm is about 10 dB/meter at a population inversion of10%. Thus, to obtain a gain of 60 dB, 6 meters of fiber are utilized. Inother embodiments, a gain of 30 dB would require 3 meters of fiber. Aslong as the fiber amplifier is uniformly inverted along its length,increases in length result in corresponding increases in gain. Incontrast to low levels of inversion associated with curves 210 and 212,if a conventional pump wavelength of 976 nm is utilized (curve 218), thegain at 1064 nm is about 60 dB/meter. Thus, in a conventional fiberamplifier a single meter of fiber would produce a gain of 60 dB, or 30dB of gain would require half a meter of fiber. However, the out-of-bandgain at 1030 nm for curve 218 is significant (˜100 dB/meter). As aresult of this high out-of-band gain in comparison to the in-band gainat 1064 nm, undesirable ASE, lasing, and the like result. As the pumpwavelength is decreased below 976 nm, the out-of-band gain continues toincrease in relation to the in-band gain. To reduce the out-of-band gainion conventional systems, techniques that generally result in increasedsystem complexity and cost are utilized. One of ordinary skill in theart would recognize many variations, modifications, and alternatives.

FIG. 3 is a simplified schematic illustration of an optical fiberamplifier with reduced out-of-band gain according to an embodiment ofthe present invention. Optical amplifier 300 amplifies a 1064 nm inputoptical pulse train 310 to produce an output optical pulse train 330 at1064 nm. Optical fiber amplifier 300 includes a length ofrare-earth-doped fiber gain medium 324. In embodiments of the presentinvention, the length of rare-earth doped fiber gain medium 324 is apredetermined length and includes, but is not limited torare-earth-doped single-clad, double-clad, or even multiple-clad opticalfibers. The rare-earth dopants used in such fibers include ytterbium,erbium, holmium, praseodymium, thulium, neodymium, combinations of theseelements, and the like. In a particular embodiment, the fiber-opticbased components utilized in constructing optical fiber amplifier 300utilize polarization-maintaining single-mode fiber.

Referring to FIG. 3, in a particular embodiment, pump laser 320 isoptically coupled to a first side of the rare-earth-doped fiber gainmedium 324 through optical coupler 322. The pump laser 320 is asemiconductor diode laser producing optical output at a predeterminedwavelength and having a predetermined spectral bandwidth. It will beappreciated that the use of a semiconductor laser source for pump laser320 will provide pump radiation with a narrow spectral bandwidth.According to embodiments of the present invention, the wavelength of thesemiconductor pump laser 320 is selected to be in the range of about1000 nm to about 1040 nm. In a particular embodiment, the pumpwavelength of pump laser 320 is 1030 nm. Merely by way of example, theoptical coupler 322 may be a Wavelength Division Multiplexer (WDM) or amulti-mode pump combiner with signal feedthrough. Such optical couplersare available from Sifam Fibre Optics, Torquay, United Kingdom.

The power of the pump laser 320 is selected to produce a substantiallyuniform inversion as a function of the length of the fiber amplifiergain medium 324. In a particular embodiment, the optical power coupledinto the fiber amplifier gain medium 324 through optical coupler 322 is500 mW. Generally, a pump laser 320 of greater than or equal to 30 500mW is utilized in this embodiment. For a fiber amplifier according to anembodiment of the present invention, the optical power absorbed by thefiber amplifier gain medium 324 is about 100 mW, resulting in about 400mW of optical pump power exiting the end of the fiber along with theoutput pulses 330 at the wavelength of the input pulses 310, forexample, 1064 nm. Thus, the optical power exiting the fiber amplifier300 at the pump wavelength is four times the optical power absorbed bythe fiber amplifier gain medium 324.

Because the population inversion and gain are substantially uniform as afunction of fiber length, the gain is linear with length, so that if atthe signal wavelength, the gain is 10 dB for a 1 meter fiber, the gainwill be 20 dB for a 2 meter fiber. This correspondence between gain andfiber length will result as long as there is significant pump leakage atthe end of the fiber. It will be appreciated that because of therelatively small portion of the pump power absorbed by the gain medium,embodiments of the present invention contrast with conventional fiberamplifiers in which significantly more pump power is absorbed in thegain medium.

Characterization of fiber amplifiers provided according to embodimentsof the present invention may be carried out using the followingprocedures. These characterization procedures are not intended to limitthe scope of the present invention, but are merely provided by way ofexample. The power of the input signal (either peak pulse power, timeaveraged power, or other measures) and the power of the output signalare measured to determine the gain at the signal wavelength, forexample, 1064 nm. The gain at other wavelengths, for example, 1030 nm,is measured using either a tunable laser source or a series of sourcesoperating at a number of wavelengths. Accordingly, the spectral gaincurve for the fiber amplifier is measured, providing data similar toFIG. 2. Depending on the pump wavelength, curves similar to the curvesillustrated in FIG. 2 will result from these measurements.

In order to determine the inversion as a function of fiber amplifierlength, power measurements of the pump laser output, couplingcoefficients to the fiber, and power measurements of optical powerexiting the amplifier at the pump laser wavelength are made. Variationsin pump power, along with other techniques, may be used to determine theuniformity of the population inversion as a function of length. At lowpump power levels, increases in pump power will result in significantportions of the pump energy being absorbed. As the pump power level israised, increases in pump power will produce a decreasing portion ofabsorbed pump energy as the gain medium becomes uniformly inverted alongthe length of the fiber amplifier. Based on the characterization of thesystem performance, comparisons may be provided between the gain at thesignal wavelength and the gain at out-of-band wavelengths.

FIG. 4A is a simplified illustration of a gain spectrum for an opticalfiber amplifier according to an embodiment of the present invention.Merely by way of example, the gain spectrum for an ytterbium-doped fiberamplifier is illustrated in FIG. 4A. The gain spectrum of theytterbium-doped optical fiber amplifier is illustrated for core pumpingat a pump wavelength of 1030 nm. The pump power for the embodimentillustrated in FIG. 4A is 500 mW. The ytterbium doping level isapproximately 5×10²⁴ ions/m³. In a specific embodiment, the length ofoptical fiber was selected to achieve approximately 22 dB of gain at thesignal wavelength of 1064 nm. As can be seen in FIG. 4A, in contrast toconventional fiber amplifier, the peak of the spectral gain curve iscentered at about the signal wavelength, thereby preventing a variety ofpotential instabilities and/or parasitic lasing at out-of-bandwavelengths. At a particular out-of-band wavelength of 1030 nm, the gainis less than 0 dB.

FIG. 4B is a simplified illustration of an output spectrum for anoptical fiber amplifier according to an embodiment of the presentinvention. As illustrated in FIG. 4B, the output spectrum illustrateslasing at the signal wavelength and minimal out-of-band ASE. Thereductions in out-of-band gain provided by embodiments of the presentinvention enable the fabrication of optical amplifiers and lasers thatare more stable and have higher gain at the useful signal wavelength.

FIG. 5 is a simplified illustration of a gain spectrum forytterbium-doped optical fiber amplifiers of various lengths. Inparticular, gain spectra are illustrated for ytterbium doped opticalfibers with lengths of 0.6 meters (510), 5 meters (512), and 10 meters(514). The optical fibers are clad-pumped with 40 W of optical power at976 nm.

The optical fiber used in generating the gain spectrum shown in FIG. 5is a double-clad fiber with a core diameter of 30 μm and an inner claddiameter of 250 μm. The ytterbium doping level is 9.2×10²⁵ m⁻³ in thecore. This ytterbium doping level is selected to provide operation inthe wavelength range around 980 nm to 1100 nm. Other rare-earthelements, like erbium, neodymium, or thulium, can be used foramplification at other wavelengths as appropriate to the particularapplications.

Referring once again FIG. 5, curve 510 for 0.6 m of doped fiber has again of about 30 dB at the signal wavelength of 1064 nm. A substantialout-of-band gain peak is present at approximately 1030 nm. Asillustrated for curve 510, the out-of-band gain peak is almost 60 dB. Itwill be appreciated that out-of-band gain peaks as shown provide a highvalue of gain and may result in parasitic lasing or instabilities in thevicinity of 1030 nm. As a result of such parasitic lasing orinstabilities, the available gain may be limited and the noise may beincreased at the wavelength of interest (1064 nm).

Embodiments of the present invention provide amplifier and/or laserdesigns that account for the presence of any such substantialout-of-band gain peak. In some conventional approaches, a technique tolower the out-of-band gain peak is to lengthen the optical fiber, whichfavors the re-absorption of short-wavelength signals, thus loweringtheir gain. This is exemplified by curves 512 and 514, which illustratethe effect of increasing the fiber length to 5 m and 10 m, respectively.However, increasing the fiber length results in increased gain at longerwavelengths. As shown in FIG. 5, increasing the length of the fiber to10 m (curve 514) results in a gain of about 50 dB at the wavelength ofinterest of 1064 nm. Therefore, some conventional techniques presentdifficulties in designing an amplifier characterized by a specific gainvalue while maintaining a desired spectral gain profile at the sametime.

Another approach used in some conventional designs is to vary the pumplevel to adjust the gain. For example, in the 10 m fiber illustrated bycurve 514 in FIG. 5, the 50 dB gain at 1064 nm could be lowered byreducing the pump power below 40 W. However, a lower optical pump powergenerally results in an increase in the recovery time of the amplifier,which may not be desirable since high pump powers are typically requiredto operate the amplifier at high pulse repetition rates. Therefore,these conventional designs present difficulties in achieving a targetgain and a fast recovery time at the same time.

FIG. 6 is a simplified schematic diagram of an optical fiber amplifierwith reduced out-of-band gain according to another embodiment of thepresent invention. Optical amplifier 610 amplifies an input opticalpulse train 620 at 1064 nm to provide an output optical pulse train 640at 1064 nm. Optical fiber amplifier 610 includes a length of rare-earthdoped fiber gain medium 638. In embodiments of the present invention,the length of rare earth-doped fiber includes, but is not limited torare-earth-doped single-clad, double-clad, or even multiple-clad opticalfibers. The rare-earth dopants used in such fibers include ytterbium,erbium, holmium, praseodymium, thulium, or neodymium. In a particularembodiment, the fiber-optic based components utilized in constructingoptical amplifier 610 utilize polarization-maintaining single-modefiber. In another particular embodiment, the fiber is doped withytterbium at a level of around 9.2×10²⁵ cm⁻³.

In particular embodiments, an optical pump 634 is coupled to a firstside of the rare-earth-doped fiber 638 through an optical coupler 636.In a particular embodiment, optical coupler 636 is a Wavelength DivisionMultiplexer (WDM) or a multimode pump combiner with signal feedthrough,which are available from, for example, Sifam Fibre Optics of Torquay,UK. According to an embodiment of the present invention, a semiconductorpump laser 634 with a wavelength in the range of 910 nm-1000 nm (e.g.,976 nm) and a power of about 40 W is utilized. Although a single pumplaser 634 lasing is illustrated in FIG. 6, one or more semiconductor orother lasers may be utilized in alternative embodiments. One of ordinaryskill in the art would recognize many variations, modifications, andalternatives. In a particular embodiment, pump laser 634 is a multi-modesemiconductor laser and the pump power is injected in theytterbium-doped fiber clad using a multimode fiber combiner 636.

According to an embodiment of the present invention, optical fiberamplifier 610 utilizes seed light from seed source 630 at a differentwavelength than the light from the pump source 634 and the input pulse620. Preferably, the seed source has a wavelength located between thepump source wavelength and the input pulse wavelength. As described morefully throughout the present specification, embodiments of the presentinvention clamp the gain peak to provide numerous benefits. Asillustrated in FIG. 6, seed light is provided by a seed source 630 thatis injected into the rare-earth-doped fiber by an optical coupler 632.In a particular embodiment, the seed light is provided by asemiconductor laser operating at a wavelength of about 1030 nm, havingan optical power between about 50 mW and about 500 mW, and is injectedinto the ytterbium-doped fiber core using a wavelength divisionmultiplexer 632. In yet another embodiment, the seed light from seedsource 630 is provided by ASE from an optically pumped ytterbium-dopedfiber amplifier and can propagate collinearly with the optical inputsignal 620.

The following discussion provides a description of a gain clampingmechanism, although embodiments of the present invention are not limitedto this particular description. When a signal is amplified strongly, itextracts energy from the gain medium and consequently the gain islowered. As applied to the optical amplifier 610, the 976 nm pump isabsorbed in the fiber and generates broad band gain at wavelengths ofboth 1030 nm and 1064 nm. The input of a strong seed signal at 1030 nmresults in the seed signal being strongly amplified, and by extractingenergy from the amplifier, lowers the gain at 1030 nm. It will beappreciated that the 976 nm pump energy in the inner clad issubstantially converted into 1030 nm light in the core, which pumps thecore of the optical fiber to provide gain at longer wavelengths,including at 1064 nm. The absorption and conversion of the 976 nm pumpenergy to pump energy at 1030 nm results in pumping effectivelyoccurring at 1030 nm. Since the effective pumping wavelength is 1030 nm,the gain at this wavelength is 0 dB for the condition in which the gainis fully inverted along the length of the fiber. Only a small portion ofthe optical fiber where the 976 nm to 1030 nm conversion occurscontributes any residual gain at 1030 nm. Therefore, any gain excess at1030 nm is minimized.

High power lasers are not as readily available at 1030 nm as at otherwavelengths. To achieve high output power from an amplifier, pump powerin excess of 50 W is often required. Referring to FIG. 6 again, in anembodiment of the present invention, a single mode continuous wave (CW)semiconductor seed laser 630 with an output power between 50 mW and 100mW at a wavelength about 1030 nm is coupled into an ytterbium-dopedfiber 638 using a Wavelength-Division-Multiplexer (WDM) 632. Theytterbium-doped fiber 638 is typically a double-clad fiber with acore-diameter of about 30 μm and an inner clad diameter of 250 μm. Seedlight from the seed laser 630 propagates into the 30 μm core of thefiber 638. Additionally, a multi-mode pump laser 634 of about 50 W at awavelength between about 910 nm and 980 nm is coupled into theytterbium-doped fiber 638 using a multimode combiner 636. The pump lightfrom the pump laser 634 propagates dominantly in the 250 μm inner cladof the fiber 638. The pump light being absorbed in the fiber stronglyamplifies the seed light resulting in a significant conversion of pumplight into seed light at 1030 nm and saturation of the gain at 1030 nm.Typically the conversion would result in about 30 W to about 40 W of1030 nm light propagating in the core. Therefore, the amplifier, insteadof being clad pumped at 976 nm is now being core pumped at 1030 nm.

A pulsed input light signal 620 at 1064 nm is amplified to provide anoutput light signal 640. It will be appreciated that although the gainis saturated at 1030 nm because of the seed laser, there is sufficientgain left at 1064 nm to amplify a signal by 30 dB by selecting apredetermined length for the ytterbium-doped fiber depending on theapplication. As the fiber length is increased, a larger fraction of thepump light is converted into seed light along with an increase in the1064 nm gain. In the current embodiment, the same double-cladytterbium-doped fiber is used simultaneously to generate core pump lightat 1030 nm and to 30 amplify a 1064 nm signal.

FIG. 7 is a simplified illustration of a gain spectrum of an opticalfiber amplifier according to another embodiment of the presentinvention. For comparison, FIG. 7 illustrates gain spectra associatedwith the same ytterbium-doped optical fiber with a doping level of9.2×10²⁵ cm⁻³ as illustrated in FIG. 5. To obtain the data illustratedin FIG. 7, a double-clad fiber with a 30 μm core and a 250 μm inner cladwas utilized. Additionally, a pump power of 40 W at 976 nm and a seedpower of 200 mW at 1030 nm was utilized. Curve 612 has a gain of around30 dB at 1064 nm using a 2.7 m fiber length. Curve 614 has a gain ofaround 45 dB at 1064 nm using a 5 m fiber length. For either spectrum,the out-of-band gain minus the in-band gain is less than 5 dB.Therefore, utilizing embodiments of the present invention it is possibleto adjust the absolute gain and to minimize the gain ripple at the sametime.

Embodiments of the present invention may be utilized in a wide varietyof applications including micro-machining, laser trimming, laserdrilling, and the like. One of ordinary skill in the art would recognizemany variations, modifications, and alternatives.

Embodiments of the present invention provide a pulsed fiber amplifierhaving its gain medium fully inverted along its length. Accordingly,independent control is available over the total amplifier gain and therecovery time of the gain between optical pulses. This fiber amplifieris particularly suited for the amplification of optical pulses withconstant pulse-to-pulse characteristics and the occurrence of firstpulse overshoot is reduced compared to conventional techniques.

During applications including marking, engraving, micro-machining,cutting, and the like, depending on the applications and the material tobe processed, the pulse characteristics of the laser, including thepulse width, the repetition rate, the peak power, and/or the energy perpulse is adapted for the task at hand. Usually, when the laser isoperated in a pulse-on-demand mode, the first optical pulse tends to bemore powerful than the following pulses. This is generally anundesirable effect. An explanation, not intended to limit embodiments ofthe present invention, is that the energy stored in the laser gainmedium is depleted significantly after the first pulse and is not fullyreplenished by the time the next pulse comes along. In other words, theoptical gain doesn't usually recover rapidly enough between pulses toprovide consistent pulse to pulse power characteristics. In somesystems, the first pulse problem is addressed using complex electroniccontrol methods.

Also, conventional fiber amplifiers are generally operated in such a waythat the total gain, the gain recovery time, and the output pulse energyare not independent of each other. Varying one parameter generallyresults in undesired variations in the other parameters. For example, toreduce the gain recovery time, one can increase the pump power, whichwould also increase the total gain available to the first pulse. Thislatter is not necessarily desirable because then one would have toeither accept an increased power first pulse, or decrease the inputpulse energy to compensate for it.

Thus, there is a need in the art for systems and techniques that providelaser amplifiers that reduce the power difference between the firstpulse and subsequent pulses.

According to embodiments of the present invention, a fiber amplifier isprovided as an optical amplifier. The methods and systems describedherein are also applicable to other amplifiers including solid-stateamplifiers such as, but not limited to, solid-state rod amplifiers orsolid-state disk amplifiers.

In a typical optical fiber amplifier, the fiber is pumped by an opticalbeam, typically a laser. In a doped fiber, this pump light is absorbedwithin the fiber by ions of the added rare-earth elements. Typically,the absorbed light causes the rare-earth ions to be excited from theirground state to a higher energy or “upper-level” state. Ions in thisupper level state are said be “inverted.” It is the inverted ions thatprovide gain to the optical signal, and the amount of gain is determinedby the proportion of rare-earth ions that are in the inverted state.This fraction is commonly referred to as “the inversion.” Depending onthe wavelength of the pump light, there is a maximum inversion that canbe attained. When this maximum inversion is reached, the optical fiberis said to be “fully inverted.” This maximum inversion decreasesmonotonically with increasing pump wavelength.

FIGS. 8A, 8B, and 8C illustrate the population inversion, the ASEintensity, and the residual pump power along a fiber length for apumping power of 20 W. FIGS. 9A, 9B, and 9C illustrate the upper-levelpopulation and the residual pump power along a fiber length for apumping power of 40 W. Referring to FIGS. 8 and 9, the inversion, theASE intensity, and the residual pump power along a length ofytterbium-doped double-clad fiber are illustrated. In the exampleillustrated the fiber length is 4 m. The ytterbium-doped, double-cladfiber has a core diameter of 30 mm and an inner clad diameter of 250 mm.The ytterbium doping level is 1.4%.

FIGS. 8 and 9 represent a condition in which the amplifier is in thesteady-state condition, with no pulses going through it. It should benoted that the inversion is not constant along the fiber length and thatthe inversion is different for the two input pump powers. For 20 W pumppower, the maximum of the inversion reaches about 0.35, whereas for 40 Wpump power the maximum of the inversion reaches about 0.40. In bothcases, the inversion is strongly depressed by the forward- andbackward-propagating ASE. As a result, the inversion is not maximizedand the fiber is not fully inverted along its length. Referring to FIGS.8 and 9, the low level of residual pump power present at the end of thefiber demonstrates that substantially all the pump power is absorbedwithin the fiber.

The amplifier pumped by 40 W will exhibit faster gain recovery betweenoptical pulses than the amplifier pumped by 20 W. Unfortunately thisfaster gain recovery happens at the expense of an increase in theinitial gain for the higher pump level. In general, for such anamplifier, it is not possible to change the gain recovery time withoutaffecting the initial gain.

FIGS.10A and 10B illustrate an example of the high energy first pulseproblem and gain recovery time for a pumping power of 20 W. FIGS. 11Aand 11B illustrate an example of the high energy first pulse problem andrecovery time for a pumping power of 40 W. Referring to FIG. 10A, fiveoptical pulses are illustrated. As illustrated in FIG. 10A, the peakpower is greater in the first pulse than in any of the subsequentpulses. Thus, the high energy first pulse problem discussed above isillustrated. FIG. 10B illustrates the gain as a function of time showinga quick decrease after each pulse and a slow incomplete recovery betweenpulses. Comparing FIG. 10B to 11B, the amplifier pumped by 40 W doesrecover more quickly than the amplifier pumped by 20 W, as illustratedby the higher slope of the gain between pulses. Also, it can be seenthat the energy of the first pulse is almost always higher than theenergy of the following pulses, in some cases by approximately an orderof magnitude. Comparing FIG. 10A to 10B, the first pulse of theamplifier pumped by 40 W is more energetic than the first pulse of theamplifier pumped by 20 W illustrating the fact that the amplifier gainis larger when pumped by 40 W than 20 W. Therefore the amplifier gainand the gain recovery time are strongly coupled together.

Embodiments of the present invention utilize gain medium, in particular,rare-earth doped fibers that are generally shorter in length thanconventional fiber amplifiers. In some embodiments, the fiber amplifiersare pumped at pump power levels higher than convention pump powerlevels. As a result of these conditions, either alone or in combination,embodiments of the present invention provide optical amplifiers thatdiffer from conventional amplifiers in at least the way the inversionand the pump power absorption behave.

FIG. 12 illustrates the upper-level population, the residual pump power,and the gain profile at 1028 nm wavelength along a fiber length for apumping power of 20 W according to an embodiment of the presentinvention. FIG. 13 illustrates the upper-level population, the residualpump power, and the gain profile at 1028 nm along a fiber length for apumping power of 40 W according to an embodiment of the presentinvention. FIGS. 12 and 13 illustrate profiles along a length ofytterbium-doped double-clad fiber similar to that used to obtain thedata shown in FIGS. 8A and 8B. In this example, the fiber length is 0.8m and the input pump powers are 20 W and 40 W, respectively. It shouldbe noted that for both input pump powers, the fiber is substantiallyfully inverted along its entire length. Also, it can be seen thataccording to embodiments of the present invention, the percentage of thepump light absorbed within the fiber is less than in conventionalamplifiers. Indeed, it can be shown that the fiber would provide asimilar inversion profile with as little as 3 W of pump power. Asdiscussed more fully throughout the present specification, the maximumvalue of the inversion is determined, in part, by the wavelength of thepump laser. Referring to FIG. 2, for the results presented in FIGS. 12and 13, the wavelength of the pump laser is 975 nm, and thecorresponding maximum value of the inversion is approximately 50%.

As discussed above, the gain available from the fiber amplifier is afunction of the population inversion. Utilizing embodiments of thepresent invention, the gain of the amplifier is fixed at somepredetermined value, and remains independent of the amount of pumppower, provided the pump power is maintained above a certain lower limit(which in the case of the example of FIGS. 12 and 13, would be ˜3 W).The value of the fixed gain is chosen by selecting an appropriate fiberlength, and would depend upon the specific application. The actual valueof the gain in decibels (dB) of an amplifier constructed according toembodiments of the present invention would vary linearly with the fiberlength for a given fiber design, which simplifies the amplifier design.

FIG. 14 illustrates gain and residual pump power as a function of theinjected pump power in an amplifier provided according to an embodimentof the present invention. For the data presented in FIG. 14, a 5 mlength of Yb-doped single-clad fiber having a doping density ofapproximately 5×10²⁴ ions/m³ was utilized. FIG. 14 presents ameasurement of the amount of residual pump power not absorbed within thefiber. The residual pump power is the amount of pump power that isemitted from the end of the fiber after passing through the fiber. Thefigure also present a measurement of the single-pass small-signal gainat a wavelength of approximately 1032 nm.

FIG. 14 demonstrates that for pump power levels greater than a certainthreshold value, for example, greater than about 50 mW, some or most ofthe pump power is transmitted through the fiber and emitted from theother end. FIG. 14 also demonstrates that at pump powers above thethreshold value, the single-pass gain is essentially clamped, forexample, for the aforementioned conditions, at approximately 28 dB. Thecorrelation between the lack of pump absorption and the clamping of thegain is believed, without limiting embodiments of the present invention,to result from the fiber being fully inverted along its length.Accordingly, as additional pump power is supplied, the inversion, andconsequently the gain, shows little to no further increase.

It can be recognized that the pump power as discussed in theseembodiments can be provided by an amplified seed signal as discussed inthe context of FIG. 6. In a particular embodiment as illustrated in FIG.6, the power of the seed 630 is high enough, for example greater than 50mW, or even greater than 100 mW, to result in a significant conversionof the power of the pump 634 into seed power, resulting in theamplification of the seed signal. The seed signal hence generated actsas pump power to the fiber amplifier. For seed power levels greater thana certain threshold value, for example, greater than about 50 mW, someor most of the seed power is transmitted through the fiber and emittedfrom the other end as illustrated previously in FIG. 14. Under theseconditions, the single-pass gain is essentially clamped, which resultsin the fiber being fully inverted along its length. Consequently incertain embodiments of the present invention, the word “pump” in FIGS.12, 13, and 14 can be replaced by the word “seed” when utilized in thecontext of embodiments illustrated in FIG. 6.

Embodiments of the present invention provide methods and systemscharacterized by substantially uniform inversion as a function of fiberlength as well as pumping at high levels (e.g., at levels wheresignificant portions of the pump power, for example, greater than 50%),are not absorbed by the active medium. Moreover, embodiments providemethods and systems that utilize pumping at wavelengths longer thanconventional systems, thereby providing gain at signal wavelengths thatis greater than gain at out-of-band wavelengths. However, theembodiments are not limited to combinations of these characteristics asthey may be provided separately or in sub-combinations.

Without limiting embodiments of the present invention, it is possible toderive the following expression for the “Critical Power.” P_(cr) in adoped fiber: P_(cr)=A_(d)E_(p)/(χ_(a)+χ_(e))Γτ where A_(d)is the area ofthe doped region of the fiber, E_(p) is the energy of a photon of pumplight, χ_(a) and χ_(e) are respectively the absorption and emissioncross-sections of the dopant in the fiber at the pump wavelength, Γ isthe confinement factor of the pump propagation mode with respect to thedopant area, and τ is the excited state lifetime of the dopant in thefiber. It is also possible to derive the following expression relatingthe inversion i in the fiber to the pump power P in terms of theCritical Power; i=i_(sat)(1+P_(cr)/P), where i_(sat) is the saturatedinversion. From this expression it can be seen that i approaches i_(sat)as P exceeds P_(cr). Based on these computations, in an embodiment, asthe pump light propagates along the fiber, the inversion will beapproximately saturated at all points where P exceeds approximatelythree times the Critical Power. Thus, as illustrated herein, if theresidual pump power exceeds approximately three times the CriticalPower, then the inversion throughout the whole fiber will beapproximately equal to the saturated inversion.

The Critical Power for the “seed” signal can be substantially differentfrom the Critical power for the “pump” signal. In particular, theconfinement factor F, and the values of the cross sections χ_(a) andχ_(e), can be significantly different in the two cases. When both “pump”and “seed” light are present in the fiber, it is possible to derive anexpression relating the inversion in the fiber to both the “pump” powerand the “seed” power in terms of the Critical Powers of each signal.From this derived expression it can be shown that if the “seed” powerexceeds approximately three times the Critical Power for the “seed”signal, and the “pump” power is less than approximately half of theCritical Power for the “pump” signal, then the inversion will beapproximately equal to the saturated inversion for the “seed”wavelength. In this circumstance, substantial conversion of pump lightto seed light can result in an amplifier whose inversion isapproximately or substantially uniform throughout the whole fiber, atthe lower saturated inversion level of the seed wavelength.

Thus, utilizing embodiments of the present invention, it is possible toadjust the amplifier recovery time by adjusting the amount of pumppower, without affecting the amplifier gain and the optical outputenergy. To increase the recovery time, increases in pump power areprovided. Additionally, embodiments of the present invention providemethods and systems in which extra pump power, which would generallyresult in the occurrence of a giant pulse in conventional amplifiers andan associated fast recovery time, will not result in significantadditional gain, thereby self-limiting the gain of the amplifier.

The following systems are included within the scope of variousembodiments of the present invention:

An optical amplifier or laser including an optical gain medium and anoptical pump having a wavelength selected such that the out-of-band gainis substantially smaller than the in-band gain.

An optical amplifier or laser including an optical gain medium and anoptical pump having a wavelength selected such that the out-of-band gainis reduced or minimized.

An optical amplifier or laser including an optical gain medium and anoptical pump having a wavelength selected such that the out-of-band gainis substantially similar to the in-band gain.

An optical amplifier or laser including an optical gain medium and anoptical pump having a wavelength selected such that the out-of-band ASEis reduced or minimized.

An optical amplifier or laser with the optical pump having a wavelengthselected such that the out-of-band ASE is reduced or minimized.

In various embodiments, the gain medium comprises a rare-earth-dopedoptical fiber, which may be a single-clad, a double-clad, or amultiple-clad structure. The optical fiber may be apolarization-maintaining fiber. The rare-earth-doped optical fiber mayinclude a combination of one or more rare-earth elements, including, butnot limited to: ytterbium (Yb), erbium (Er), neodymium (Nd), thulium(Th), holmium (Ho), or praseodymium (Pr). The pump comprises asemiconductor diode laser, a fiber laser, a solid-state laser,combinations of these, and the like.

An optical amplifier or laser comprising a first length ofytterbium-doped optical fiber and an optical pump having its wavelengthselected substantially in the range of 1000 nm to 1040 nm.

An optical amplifier or laser including a first length ofytterbium-doped optical fiber and an optical pumping means having itswavelength selected substantially in the range of 1020 nm to 1040 nm.

An optical amplifier or laser including a first length ofytterbium-doped optical fiber and an optical pump having its wavelengthselected substantially in the range of 1025 nm to 1030 nm.

In various embodiments, the optical amplifier or laser includes anytterbium-doped fiber comprising a single-clad fiber, a double-cladfiber, or a multiple-clad fiber. The ytterbium doping concentration maybe in the range 1×10²⁴-1×10²⁶ ions per m³. The optical fiber may be ofthe polarization maintaining type. The pump may be a semiconductor diodelaser, a fiber laser, a solid-state laser, combinations thereof, and thelike.

The following systems are also included within the scope of variousembodiments of the present invention:

An optical amplifier or laser including a first length of erbium-dopedoptical fiber and an optical pump having its wavelength selectedsubstantially in the range of 1490 nm to 1535 nm.

An optical amplifier or laser including a first length of erbium-dopedoptical fiber and an optical pump having its wavelength selectedsubstantially in the range of 1500 nm to 1530 nm.

An optical amplifier or laser including a first length of erbium-dopedoptical fiber and an optical pump having its wavelength selectedsubstantially in the range of 1515 nm to 1525 nm. The erbium-doped fibermay include a single-clad fiber having a erbium doping concentration inthe range 1×10²⁴-1×10²⁶ ions per m³. The optical fiber may be of thepolarization maintaining type. The pump may be a semiconductor diodelaser, a fiber laser, a solid-state laser, combinations thereof, and thelike.

An optical amplifier including an optical pump adapted to operate at apumping wavelength, an optical signal adapted to be amplified at asignal wavelength, and a gain clamp seed adapted to operate at a seedwavelength. In an embodiment, the optical pump, the optical signal, andthe gain clamp seed are all concurrently injected in an optical gainmedium.

An optical amplifier including an optical pump adapted to operate at apumping wavelength, an optical signal adapted to be amplified at asignal wavelength, and a gain clamp seed adapted to operate at a seedwavelength. The optical pump, the optical signal, and the gain clampseed are all concurrently injected in an optical gain medium such thatthe optical power of the gain clamp seed is operable to limit theoptical gain outside of the signal wavelength to a level less than 5 dBabove the gain at the signal wavelength. According to variousembodiments, the optical amplifier includes a fiber amplifier, which maybe a rare-earth-doped fiber gain medium, the optical pump includes oneor more semiconductor lasers, and the gain clamp seed includes one ormore semiconductor lasers.

The rare-earth-doped optical fiber may include ytterbium, erbium,thulium, holmium, praseodymium, or neodymium. The rare-earth-doped fibermay be of the single-clad type, double-clad type, or multi-clad type.

An ytterbium-doped optical fiber amplifier including an optical pumpcomprising at least one semiconductor laser at a pumping wavelengthbetween 910 nm and 1000 nm, an optical signal adapted to be amplified,the optical signal having a wavelength between 1050 nm and 1100 nm, anda gain clamping semiconductor seed having a wavelength between 1000 nmand 1050 nm. The optical pump, the optical signal, and the gain clampingsemiconductor seed are all injected into a common length ofytterbium-doped fiber.

The optical fiber amplifier further includes an optical coupler adaptedto couple the pump light, the signal light and the gain clampingsemiconductor seed light into the common length of ytterbium-dopedfiber. The optical gain outside of the signal wavelength is less than 5dB higher than the optical gain at the signal wavelength according to anembodiment. The gain clamping semiconductor seed is characterized by apower higher than 10 mW in another embodiment. The ytterbium-doped fibermay be of the single-clad type, double-clad type, or multi-clad type andmay further include other doping elements.

The following systems are additionally included within the scope ofvarious embodiments of the present invention:

An optical amplifier including a pump and a gain medium, wherein thegain medium is substantially fully inverted along its length. The pumpmay be a semiconductor diode laser, a fiber laser, a solid-state laser,combinations of these, or the like. The gain medium may include arare-earth-doped optical fiber, with either a single-clad, double-clad,or multiple-clad structure. The optical fiber may include apolarization-maintaining fiber. The rare-earth-doped optical fiber mayinclude a combination of one or more rare-earth elements includingytterbium (Yb), erbium (Er), neodymium (Nd), thulium (Th), holmium (Ho),or praseodymium (Pr).

While the present invention has been described with respect toparticular embodiments and specific examples thereof, it should beunderstood that other embodiments may fall within the spirit and scopeof the invention. The scope of the invention should, therefore, bedetermined with reference to the appended claims along with their fullscope of equivalents.

The invention claimed is:
 1. A method of operating an ytterbium-dopedfiber amplifier, the method comprising: providing an input signal at awavelength between 1050 nm and 1090 nm; providing pump radiation at awavelength between 1010 nm and 1050 nm; coupling the pump radiation toan ytterbium-doped fiber having a length in the ytterbium-doped fiberamplifier; and amplifying the input signal at the wavelength of theinput signal to generate an output signal, wherein the gain at thewavelength of the input signal in the ytterbium-doped fiber is a linearfunction of the length of the fiber.
 2. A method of operating anytterbium-doped fiber amplifier, the method comprising: providing aninput signal at a wavelength between 1050 nm and 1090 nm; providing pumpradiation at a wavelength between 1010 nm and 1050 nm; coupling the pumpradiation to the ytterbium-doped fiber amplifier; and amplifying theinput signal to generate an output signal; wherein a value of pump powerexiting an output end of the ytterbium-doped fiber amplifier is at leastgreater than or equal to three times a critical power.
 3. A method ofoperating an ytterbium-doped fiber amplifier, the method comprising:providing an input signal at a wavelength between 1050 nm and 1090 nm;providing pump radiation at a wavelength between 1010 nm and 1050 nm;coupling the pump radiation to the ytterbium-doped fiber amplifier; andamplifying the input signal to generate an output signal; wherein avalue of pump power exiting an output end of the ytterbium-doped fiberamplifier is at least greater than or equal to a value of pump powerabsorbed in the ytterbium-doped fiber amplifier.
 4. The method of claim1, wherein the pump radiation is characterized by a wavelength between1020 nm and 1040 nm.
 5. The method of claim 1, wherein the pumpradiation has a wavelength and power which produces a substantiallyuniform population inversion as a function of ytterbium-doped fiberamplifier length.
 6. The method of claim 5, wherein the substantiallyuniform population inversion is within 10% of a peak inversion value. 7.The method of claim 1, wherein the input signal is characterized by awavelength of 1064 nm.
 8. The method of claim 2, wherein the pumpradiation is characterized by a wavelength between 1020 nm and 1040 nmand the input signal is characterized by a wavelength between 1060 nmand 1070 nm.
 9. The method of claim 2, wherein the pump radiation has awavelength and power which produces a substantially uniform populationinversion as a function of ytterbium-doped fiber amplifier length. 10.The method of claim 9, wherein the substantially uniform populationinversion is within 10% of a peak inversion value.
 11. The method ofclaim 2, wherein the input signal is characterized by a wavelength of1064 nm.
 12. The method of claim 3, wherein the pump radiation ischaracterized by a wavelength between 1020 nm and 1040 nm and the inputsignal is characterized by a wavelength between 1060 nm and 1070 nm. 13.The method of claim 3, wherein the pump radiation has a wavelength andpower which produces a substantially uniform population inversion as afunction of ytterbium-doped fiber amplifier length.
 14. The method ofclaim 13, wherein the substantially uniform population inversion iswithin 10% of a peak inversion value.
 15. The method of claim 3, whereinthe input signal is characterized by a wavelength of 1064 nm.